The present invention relates to fluorescent display devices and in particular to fluorescent display devices in which phosphor material such as ZnO:Zn or the like, coated on conductive elements is selectively made to fluoresce by selective energization or deenergization of the conductive elements.
There are two broad classifications of fluorescent display devices namely (a) gas discharge and (b) vacuum fluorescent. In a gas discharge device, an electric field is set up between phosphor coated shaped cathode segments and one or more anodes in an atmosphere of easily ionizable gas such as neon. The shaped cathode segments may form characters on which the phosphor coating is selectively illuminated by applying voltages to selected ones of the segments. A typical construction of a gas discharge device is shown in U.S. Pat. No. 3,902,003.
In a vacuum fluorescent device, an electron emitter, such as a thermionic or field emitter, supplies electrons through a vacuum to phosphor coated shaped anode segments. The shaped anode segments may be selectively illuminated in a manner similar to the cathode segments by selectively applying voltages thereto, although the polarities and magnitudes of the voltages will differ between devices. A typical construction of a vacuum fluorescent display device is shown in U.S. Pat. No. 3,986,760.
Additional electrodes may be used in either type of device for simultaneously controlling groups of segments, shielding against external electric fields or cancelling the effect of charge buildup on dielectric materials of the devices.
Modern fluorescent display devices employ a generally planar insulating substrate having the conductive shaped anode or cathode segments formed on the planar surface or slightly recessed therein. Typically a plurality of changeable characters, such as 7-segment numeric patterns, are disposed on the substrate. A concave cover plate, usually of glass, is sealed at its perimeter to the substrate to form a sealed enclosure enclosing the anode or cathode segments as well as additional electrodes. The atmosphere in the sealed enclosure is evacuated or replaced with gas as required by the particular type of display device.
The energization of the segments is normally controlled by the electronic equivalent of a single-pole single-throw switch. That is, each switch either connects an energizing voltage to its associated segment or disconnects the segment thus allowing it to float.
The anode or cathode segments are usually applied to the substrate by silk screen printing, lithography or by chemical etching of a continuous conductive coating or by other methods well known in the art. Adjacent segments are usually separated by a gap of, for example, 15-20 thousandths of an inch to avoid inter-segment short circuits. The phosphor material, typically zinc activated zinc oxide, ZnO:Zn, europeum activated tin oxide, SnO:Eu, or other phosphor known in the art, is applied through a mask in order to separately cover the segments without coating the substrate between segments. Care is necessary to avoid coating the substrate between segments because the ZnO:Zn, although a poorly conducting material, is a sufficiently good conductor to provide a conductive bridge between adjacent segments. This conductive bridge is sufficiently conductive to illuminate a floating unenergized segment adjacent to an energized segment. Thus the gaps in phosphor are essential for selective illumination of the segments.
Unfortunately, the necessity for gaps between adjacent segments also results in a discontinuity in the illuminated line when adjacent segments are energized. This discontinuity interferes with accurate representation of alphanumeric characters. In addition, the care required in accurately coating the segments to avoid forming the conductive bridges increases manufacturing cost.